Ligand- and Cation-Exchanged Colloidal Quantum Dot Thin Films and Devices

Colloidal quantum dots (QDs) are nanometer-scale crystals of inorganic semiconductors that are capped by organic or inorganic ligands. These QDs form an excellent, solution-processable materials class for thin-film electronics and optoelectronics. In this talk, I will describe and compare post-synthesis surface- and cation-exchange processes to engineer the optoelectronic properties QD thin films. In the first example, starting with electronically-coupled, thiocyanate-capped, wurtzite CdSe QD thin films, we use sequential cation exchange processes to substitute Cu⁺ for Cd²⁺to realize Cu₂Se nanocrystal thin-film intermediates. Subsequent partial cation exchange with the liquid-coordination-complex trioctylphosphine-indium chloride yields n-doped CuInSe₂ NC thin films [1]. These CuInSe₂ nanocrystal films are used to form the channel of field-effect transistors (FETs), which upon Al₂O₃-encapsulation, are air-stable devices with high electron mobilities of ~10 cm²/Vs and current modulation of 10⁵, comparable to those of high-performance Cd- and Pb-containing QD FETs. In the second example, we begin with epitaxially-fused PbSe QD thin films, and using sequential Cd⁺ and Cd²⁺ cation exchange, realize epitaxially-fused, zinc-blende CdSe QD films [2]. We study the detrimental influence of Pb²⁺ impurities inherited from the starting QDs on the characteristics of CdSe QD FETs and how engineering of the device architecture can suppress these detrimental effects [3]. We realize FETs with high electron mobilities of 35 cm²/Vs and current modulation of 10⁶, after doping [2,3].<br/>1. H. Wang, D. J. Butler, D. B. Straus, N. Oh, F. Wu, J. Guo, K. Xue, J. D. Lee, C. B. Murray, C. R. Kagan, “Air-Stable CuInSe2 Nanocrystal Transistors and Circuits via Post-Deposition Cation Exchange,” ACS Nano 13, 2324-2333 (2019).<br/>2. Q. Zhao, G. Gouget, J. Guo, S. Yang, T. Zhao, D. B. Straus, C. Qian, N. Oh, H. Wang, C. B. Murray, C. R. Kagan, “Enhanced Carrier Transport in Strongly Coupled, Epitaxially Fused CdSe Nanocrystal Solids,” Nano Lett. 21. 3318-3324 (2021).<br/>3. Q. Zhao, S. Yang, J. J. Ng, J. Xu, Y. C. Choi, C. B. Murray, C. R. Kagan, “Impurities in Nanocrystal Thin-Film Transistors Fabricated by Cation Exchange,” J. Phys. Chem Lett. 12, 6514-6518 (2021).